This is not an issue with 10.8 nor any other OS X version, this is you misunderstanding OS X and file sharing.

If the permissions on the folder are set to anything other than Read/write you need to input a Password, you can see in the sharing Preference folder, if you want a shared folder to be shared without a user needing to input any name and password you can do so by setting it up the right way.

The wierdest thing is that we've setup the network sharing the same way on all the iMacs we have. But this looks like only our 10.7 is working without a user/pass prompt.

Our folders are set to R/W for everyone and using SMB.

I just tried and have no problem connecting to my Mini which is on 10.8.3 without inputting a password, I even shared the whole Macintosh HD and was not asked for a password if I set everyone to Read/write.
Must be something wrong there.

This is a not uncommon issue. There are post about it in this forum. I have it with my new iMac with 10.8.3. The solution is to go into network preferences, and disable, then re-enable SMB sharing. Once done it will stick until the Mac is rebooted.